Apolipoprotein (apo) A-I is the major apoprotein of plasma high density lipoproteins (HDL). Plasma lipoproteins play pivotal roles in cholesterol metabolism and are important factors for identifying those people at risk for atherosclerosis, stroke and cardiovascular disease. Individuals with high levels of circulating HDL are at lower risk for the complications of atherosclerosis. Numerous functions of apo A-I have been identified that contribute to its beneficial properties. These include the capacity of apo A-I to: 1) activate the enzyme that esterifies cholesterol in plasma, lecithin: cholesterol acyl transferase (LCAT), b) mediate cholesterol delivery to steroidogenic tissues, c) facilitate cholesterol egress from certain nonhepatic peripheral cells such as the cholesteryl ester loaded macrophage (e.g., the foam cells of the atherosclerotic lesion), and d) enhance fibrinolysis. Thus, apo A-I plays an important role in vascular biology and cholesterol metabolism. The primary structure of apo A-I is known, however, specific regions of apo A-I that are responsible for carrying out these functions are largely unknown. Using an immunochemical approach, the relationship between apoprotein structure and function will be explored. Four functions will be studied including LCAT activation, cholesterol delivery to steroidogenic cells, cholesterol unloading from macrophages, and the enhancement of fibrinolysis. An existing panel of apo A-I-specific monoclonal antibodies, which are directed towards unique regions on apo A-I, will be screened for their ability to block each of these apo A-I functions. When antibodies are found that interfere with a particular function, the fragments that represent the functionally important regions of apo A-I will be tested for their ability to either mimic or block apo A-I function. These studies will verify the hypothesis that specific structural regions on apo A-I can be identified that are responsible for each of its many functions. Furthermore, the successful completion of these studies will begin to provide detailed information regarding the specificity mechanism and physiologic importance of the activation of LCAT, cellular interactions, and the enhancement of fibrinolysis by apo A-I.